This invention relates to optical recording and reproducing system using localized characteristics change (real amplitude reflection coefficient, transmissivity change, deflecting direction change) due to radiation of an optical spot.
In a recording and reproducing system for a filing optical disk in accordance with the prior art technique, for example, grooves for tracking are disposed on the disk surface in its radial direction and an optical spot is radiated on or between the grooves so as to record, reproduce or erase the information data formed on or between the grooves. The grooves are used as a tracking guide for tracking of the optical spot. To reduce recording and reproducing noise, the width between the grooves (or track gap) is preferably smaller than the diameter of a recording pit for recording the data on the disk. However, the following two critical problems must be overcome in order to make the track gap in the radial direction of the disk smaller than the spot diameter and thus to accomplish high track density.
The first is the problem of tracking. The so-called "push-pull system" which is known in the art is generally used to read the optical spot diameter in the radial direction. This system utilizes the action of diffraction grating having a groove structure disposed periodically in the radial direction of the optical spot. If the track gap or the groove gap is reduced in order to attain a high density memory capacity, there occurs the problem in that it becomes difficult to detect the position of the optical spot in the radial direction because the spatial frequency (which is expressed by the inverse number of the period P of the groove) in the radial direction of the disk becomes close to the cut-off frequency of a reading optical system (which is expressed by the inverse number of the diameter of the optical spot).
The second problem is the cross-talk that occurs due to the mixture of unnecessary information from adjacent tracks. If the track gap is made smaller than the optical spot diameter, the optical spot covers the adjacent tracks even when the optical spot tracks accurately the center of the target track, and then unnecessary information will mix into the information of the target track or the cross-talk will increase.
As the method of solving the first and second problems described above and attaining the high density in recording and reproducion in the radial direction, there is known a method according to which the tracks on which the recording pits are formed have a periodical rise-and-fall construction in the radial direction as will be later described and the information from the adjacent tracks are cut off or separated optically. In the tracking method in this case, the periodical rise-and-fall construction is used either as the diffraction grating for recording or reproducing the information or as guiding grooves, or to add signal pits for guiding.
In Japanese Patent Laid-Open No. 57-105828, the disk has a V-shaped or inverted trapezoidal sectional shape 64 as shown in FIG. 7, an optical beam spot is radiated to the slope 65 of the groove and pits 66 are formed for recording and reproduction. According to this method, the gap of the information recording and reproduction position in the radial direction of the disk can be reduced to about the half of the optical beam spot diameter, but the following problem is yet left unsolved.
In order to produce the master disk having the V-shaped or trapezoidal sectional shape in the radial direction as described above, the prior art technique of this reference cuts mechanically a metal sheet by use of a diamond needle whose tip has either the V- or trapezoidal shape. To finish the surface of the groove slope, on which the recording pits are to be formed, to a high quality mirror surface, however, a cutting system which is employed ordinarily and features in a miniature structure or a system which forms the grooves by exposing a laser beam to a photoresist and makes development must be employed. However, it is difficult to produce the master disk described above by ordinary laser cutting.